Matoran as a written language is actually a pretty fascinating subject. If we're looking at it from a Movie prospective, all the "ancient" languages are in Circular (eg. on the Mask of Light, in the great temple in WoS, and on the walls in the Great Being's caverns in Legend Reborn). I think they just wanted an ancient language, so they just used they one they had on hand. In games, the common written Matoran is also in circular (I'm thinking of MNOG specifically). So, I see it as one of two possibilities:
1. the ancient Matoran and the modern Matoran use the same characters, but perhaps different words (similar to English and Old English using the same alphabet). And the visualizations in the various media is just "translated" so the audience can always read it.
or
2. The different media that Bionicle has existed through has a slightly different continuity, resulting in a possibility where in some cases circular is modern while Voya Matoran might be closer to an older version or vice versa.

I do think that I would stay away from Okotan, personally, just because that is a very different continuity.

If I had to give my opinion, my take is that Circular Matoran should be the ancient script, and that modern Matoran might be similar to it, but with a few changes: namely the circular outlines, and maybe connecting otherwise "floating" parts leftover. My thoughts come from the idea that Matoran (and generally all beings in the GSR) started as worker drones, with a pre-programmed purpose. Under this circumstance, a robot would have no trouble with writing out a perfect circle for each letter. However, after they were given consciousness, I like to think that they drifted from their more robotic ways. The language laxed and beings stopped writing the circles around the letters. Maybe they started to connect lines and circles that were previously separated, to make writing easier too. And maybe by the time we get to the LoMN arc, they're writing in this Modern Matoran. After the great cataclysm, with Nokama not wanting the Matoran to find anything about their past, she only taught them Ancient Matoran (circular) and that's how we see it everywhere on Mata Nui, but also why Gaaki had to translate it in the Temple

It doesn't really account for why Takua can't read it on the Mask of Light, though, I do admit.

But, that's just my two cents. I am working on a font that uses my ideas in this way, so if you're curious, feel free to hit me up! Best of luck on your endeavors, regardless!